On Ubuntu 14.04 LTS, a2ensite automatically appends ".conf" to the filename it looks for in /etc/apache2/sites-available/
Therefore, the file "/etc/apache2/sites-available/roundcube" must be renamed to
"/etc/apache2/sites-available/roundcube.conf".

Security issue:
This file must be owned by root, otherwise it is a huge security issue (User www-data could modify the file and get root at next restart of apache).